Heretofore, in various fields, there have been utilized techniques for variously treating a surface of a workpiece by using a plasma in this way. Conventional plasma surface treatment methods have problems in that generally, special equipment and facilities, such as a vacuum chamber, are required to cause a plasma discharge in a reduced pressure environment, the performance of a surface treatment is low and it is difficult to treat a surface of a large area. As a result, the manufacturing cost becomes high. Thus, recently, there have been proposed surface treatment techniques as described in, for example, the Japanese Patent Application No. 6-2149 Official Gazette, which can obviate the necessity of a vacuum facility and can simplify and miniaturize the equipment and enable the treatment of a surface of a large area at a low cost.
An example of a conventional apparatus for treating a surface of a large area by using an atmospheric-pressure plasma in this way is schematically illustrated in FIG. 12. This surface treatment apparatus 1 has a disk-like upper electrode 3 and a disk-like lower electrode 6 which are placed a certain distance apart in parallel with each other in a housing 2. The lower electrode 6 is grounded and a workpiece to be treated 4 is put thereon over a dielectric 5. The upper electrode 3 is connected to a power supply 7, and a porous dielectric 8 is placed thereunder so as to do away with an abnormal discharge and obtain uniform distribution of a discharge gas. A discharge is caused between both of the electrodes by applying a predetermined voltage to the upper electrode 3 from the power supply 7.
The upper electrode 3 is provided with many air holes (not shown) bored therein in such a manner as to provide an opening to the dielectric 8. Gas supplied from an external gas supply 10 to a chamber 9 defined therein is introduced through the air holes the dielectric 8 into a discharge region 11 in the housing 2. The gas is dispersed by the porous dielectric 8 and is supplied uniformly to the entire discharge region, so that a plasma is uniformly produced in the discharge region 11. The surface of the workpiece 4 is treated by exposing excited active species of the gas generated from this plasma. The gas flows from the periphery of the discharge region 11 to the bottom side of the upper electrode 6 and is then exhausted from a gas exhaust port 12 provided in the central part of the bottom portion of the housing 2, as indicated by arrows in FIG. 12.
FIG. 13 schematically illustrates another conventional surface treatment apparatus 13. This surface treatment apparatus 13 is different from the aforementioned surface treatment apparatus of the face type and is what is called a line type that treats the surface of the workpiece, which relatively moves just thereunder, while scanning the surface thereof. Further, this surface treatment apparatus 13 has an elongated electrode 14 extending in a direction indicated by an arrow A in this figure, which is orthogonal to the direction of the movement of the workpiece. Two discharge generating portions 15 and 16 of the same size and shape are provided on the bottom surface of the electrode 14 in such a way as to extend in parallel with each other over the full length of the electrode 14 and project downwardly from the bottom surface thereof. Further, a dielectric 18 is fitted to the bottom portion of the electrode 14 between both of the discharge generating portions in such a way as to define an intermediate chamber 17 over the full length of the electrode 14. Gas ejecting ports 19, each of which opens downwardly from the intermediate chamber 17, are linearly formed in the dielectric 18 over the full length of the electrode 14. Further, gas inlets 20 opening upwardly in the top surface of the electrode 14 communicate with the intermediate chamber 17. When a predetermined voltage is applied to the electrode 14, an electric discharge is caused among both of the discharge generating portions 15 and 16 and the workpiece which pass through thereunder. Gas supplied from the external gas supply through the gas inlets 20 into the intermediate chamber 17 is ejected from the gas ejecting ports 19 downwardly to the workpiece and is then introduced into the plasma produced by the discharge. Thus, excited active species are generated to thereby treat the surface of the workpiece.
However, in the apparatuses of FIGS. 12 and 13, the following problems have arisen. First, in the case of the conventional surface treatment apparatus of FIG. 12, the porous dielectric 8 is directly fixed to the housing by fixation means such as screws 21. Thus, there are concerns that cracks and warpage are caused by local stress caused therein and thermal stress, owing to the high temperature of a plasma discharge; and that if a crack or warpage occurs in the porous dielectric 8, the desired function of uniformly distributing the gas over the discharge region cannot be performed; conversely, if the output of the power supply is suppressed in order to prevent the temperature of the discharge plasma from becoming excessively high, a sufficient discharge is not obtained, the performance of a surface treatment is lowered and thus the treatment rate becomes reduced or the surface treatment is not sufficiently achieved; and that further, if a crack is produced in the dielectric 8, the function of preventing an occurrence of an abnormal discharge, which is the essential function of the dielectric 8, is impaired.
Further, in the case of this conventional apparatus, even though the distribution of the gas in the discharge region is made to be uniform by using the porous dielectric, the exhaust is performed only from the single gas exhaust port provided on the back side of the lower electrode, as above described. Thus, when a certain gap between the dielectric 8 and the workpiece to be treated is not maintained owing to a mounting error or the like of the apparatus, there are concerns that a deviation or deflection of a gas flow is caused, that an irregularity or nonuniformity is caused in the distribution of the gas in the discharge region 11 and that the entire workpiece cannot be treated uniformly. Further, there are additional concerns that organic substances and the like removed from the workpiece can be redeposited thereon during ejection from the discharge region and that thus, the workpiece can become contaminated and the maintenance of the apparatus can be difficult.
Moreover, it is favorable for realizing a uniform distribution of the gas that a porous plate is used as the upper electrode 3 instead of providing many air holes therein. In contrast, in the case of using a carbon material or SUS base material, such a material reacts with gases such as O.sub.2 and CF.sub.4, so that oxidization and corrosion are liable to occur. Therefore, there is a concern that a detached material from the electrode can contaminate the surface of the workpiece and the interior of the apparatus. Especially, in the case that the workpiece to be treated is a silicon wafer, metal contamination by, for example, Fe can be a problem. Furthermore, the need for periodic replacement of such an electrode material arises from the fact that such an electrode material is a consumable good. As a result, the operating cost is increased. Eventually, the cost of the apparatus becomes high.
Further, in the case of the conventional surface treatment apparatus of FIG. 13, it is difficult to cause an electric discharge uniformly in the longitudinal direction of the electrode under near atmospheric pressure even if the width of the discharge generating portion is made to be uniform, which is different from the case of a vacuum discharge. Moreover, because of the problem of an abnormal discharge, there is a limit to the power for causing an electric discharge. A discharge area, which depends upon the width of the discharge generating portion, results in a fixed certain range of the discharge region. Thus, neither the enlargement of the discharge region nor the regulation or enhancement of the performance of the surface treatment can be performed according to treatment conditions. Consequently, especially, in the case of treatment of a large area, it is difficult to achieve a uniform treatment. Moreover, the realization of high-speed treatment cannot be expected.
Furthermore, the intermediate chamber 17 is used for making uniform the pressure of the gas supplied from the gas inlets 20 of a limited number in the longitudinal direction of the electrode, to thereby cause the gas to be uniformly ejected from the full length of the gas ejecting ports 19. However, because of the fact that each of the gas ejecting ports 19 and a corresponding one of the gas inlets 20 are placed in line, most of the gas introduced from each of the gas in lets 20 flows directly into the corresponding gas ejecting port 19. Thus, there are concerns that the distribution of the gas ejected in the longitudinal direct ion of the electrode becomes nonuniform and that uniform surface treatment cannot be achieved and consequently, the enhancement of the treatment performance cannot be accomplished.
Besides, in the case of this conventional apparatus, the gas supplied from the gas ejecting ports 19 to the discharge region is exhausted by being easily dispersed into the air from the front and rear of the electrode when viewed from the direction of the movement of the workpiece. Regarding the gas, helium, which is relatively expensive, is mixed in to reactive gas in order to stabilize the electric discharge. This conventional apparatus, however, has a problem in that when the flow rate of helium is increased correspondingly to the dispersion of the gas, the treatment cost is increased considerably. Further, there are concerns that because air is mixed in the discharge region when the gas is dispersed, that the discharge will become unstable, or the organic substances or the like, which have been removed, will react with impurities contained in the air and become redeposited onto the surface of the workpiece. To effectively prevent this and stabilize the discharge, the apparatus requires an appropriate exhaust mechanism to reduce the concentration of gaseous impurities which a recontained in the air, in the discharge region. Moreover, from the viewpoint of environments protection, it is necessary to suitably treat the exhaust gas, including ozone or the like, which is generated by the plasma discharge.
Furthermore, in the case of such a surface treatment apparatus, regardless of the electrode configuration, even when the dielectric for protecting the electrode is provided therein, there are concerns that abnormal discharges such as an arc discharge are caused and thus, that the workpiece and a stage on which the workpiece is placed, will be damaged. Consequently, such an apparatus has problems in that the output thereof is limited, that therefore, the treatment performance cannot be sufficiently enhanced and neither high-speed treatment nor reduction in cost can be achieved.
Thus, the pre sent invention is accomplished in view of the aforementioned problems of the prior art. An object of the present invention is to provide a surface treatment apparatus, which is called the face type, adapted so that the gas supplied to the discharge region passes through the plate-like electrode so as to perform a surface treatment by using a plasma generated under a near atmospheric pressure, and which can uniformly produce a plasma by making the distribution of the gas contained in the discharge region uniform to thereby uniformly treat a large surface area and to achieve the enhancement of the treatment performance and the reduction in cost thereof. In addition, particularly, another object of the present invention is to realize a uniform distribution of the gas at all times without causing damages or the like to the porous element even in the case that an output of the power supply to be applied to the electrode is increased in a surface treatment apparatus in which the porous element is placed on a side on which an electric discharge is caused by the plate-like electrode, thereby increasing the speed of the surface treatment and enhancing the treatment performance.
Further, an object of the present invention is to provide a porous element electrode, which is used in the surface treatment apparatus, adapted so that the gas supplied to the discharge region passes through the electrode, and by which the distribution of the gas contained in the discharge region can be made uniform, and which has high durability and is in no danger of contaminating the workpiece by being subject to oxidation, corrosion and the like due to the gas species used in the apparatus, and which can reduce the operating cost thereof.
Moreover, another object of the present invention is to provide a surface treatment apparatus, which is called the line type, adapted so that it performs a surface treatment by using a plasma generated under a near atmospheric pressure when scanning the workpiece with the elongated electrode linearly extending when the workpiece performs a relative movement in a direction orthogonal to the longitudinal direction of the electrode, and which can regulate a discharge area along the longitudinal direction of the electrode and thereby can regulate the treatment rate of each part of the electrode according to the treatment conditions and which can enhance the treatment performance over the full-length of the electrode, thereby enabling the high-speed treatment of a large surface area.
Furthermore, the present invention aims at realizing the uniform distribution of the supplied gas over the full-length of the electrode in the surface treatment apparatus of the line type, thereby making the plasma uniform, enhancing the treatment performance and achieving the treatment of a large surface area.
Still another object of the present invention is to provide an efficient compact exhausting mechanism of a surface treatment apparatus of the line type, which can suppress the dispersion of a gas into the air, by limiting the exhaust path of the gas that has been supplied to the discharge region and which can prevent the air from being mixed in the discharge region and can stabilize the plasma discharge and the surface treatment utilizing the plasma discharge, and which can improve the treatment performance and reduce the treatment cost.
Additionally, yet another object of the present invention is to provide a surface treatment apparatus which uses a plasma under a near atmospheric pressure and which can eliminate the fear of occurrence of abnormal discharges such as an arc discharge among the electrode, the workpiece and the stage according to the discharge conditions regardless of the aforementioned electrode configuration and thereby can obtain a high output thereof and enhance the treatment performance thereof.